1. Field of the Invention
This invention relates to printing apparatus and, more particularly, to magnetic means for controlling the application of a toner to a transfer surface prior to printing.
2. Description of the Prior Art
Printing methods which employ an electrical field to move particulate printing material, toner, to a recipient sheet are known in the prior art. One such printing process employs a printing sheet and a transfer surface or belt from which the toner is conveyed to a print head which establishes an electrical field for transferring the toner material to the recipient sheet or paper. The imprinted paper passes over a hotplate of sufficient temperature and length to fuse the paper and toner. A method of supplying toner to the recipient sheet that has been considered envisions using a permanently toned ribbon which would be used only once and then discarded. As this scheme is generally not practical, a continuous printer having a retoneable transfer surface or belt in contact with the toner, preferably a toner bead, for coating the belt has been considered. The rotating toner bead is comprised of fine magnetic particles and is formed by the magnetic field gradients of a plurality of magnets employed above and below the bead and on opposite sides of the belt.
In general, the rotating magnetic bead scheme works well at low belt velocities, that is, on the order of 20 inches per second, and when used with a rough surface belt. Present printing requirements, however, indicate that printing speeds on the order of 50-60 inches a second or more are desirous. However, as the rotating magnetic bead tends to fly apart at belt velocities on the order of 30 inches per second, a 60 inch per second belt velocity would require a bead four times larger in diameter and magnets eight times more massive (assuming the same magnetic material) than those originally used in the 20 inch per second system. Furthermore, at these faster speeds, uniform coating of the transfer surface is difficult.
Accordingly, there is a need for providing means for operating a non-impact printer at increased velocity which uniformly coats the transfer surface with sufficient toner for good printing characteristics.